Abstract: A recent occultation
observing campaign inadvertently captured a meteor in multiple narrow
field-of-view video cameras. A quick analysis of the tracks classified it as a
sporadic with implications for the possibility of doing short baseline meteor
triangulation. The primary goal of the collection was also met in that the size
and shape of asteroid Metis was successfully measured.

Introduction

The following
report is about a serendipitous recording of a meteor on multiple video cameras
that occurred during the asteroid Metis occultation timing experiment. The
principle interest of one of the authors (SD) is in designing inexpensive narrow
field of view camera systems and deploying them for asteroid occultations of
stars to support the measurement of asteroid size and shape. The chance meteor
appearance during a recent collection campaign, however, suggests an alternative
way of doing meteor triangulation with short baseline observations. Thus this
paper describes the event, the observing method and equipment, and data
reduction results which could prove to be inspirational to others in the meteor
community.

Event Background

On the morning of
September 12, 2008 the asteroid (9) Metis was predicted to pass in front of the
6th magnitude star HIP 14764 and would be visible from a 271km wide
path on the ground across the United States. The International Occultation
Timing Association (IOTA) scheduled its annual meeting to occur on the same date
in Apple Valley , California to get as many people together in one general
location and thus provide as many chords of occultation measurements as possible
across the asteroidal body. The California location was chosen for its stable
and generally clear weather during that time of year. For the Metis event, the
plan was to deploy for the first time, fifteen of “Mighty Mini” observing
systems that will be described in detail below. These fifteen unattended
systems were to be spaced approximately 3km from each other along a line roughly
perpendicular to the ground path of the shadow. They were all pre-pointed to the
same exact altitude and azimuth in the sky so that at precisely 06:21:59 UT, the
star HIP 14764 would drift through the center of the field of view of each
camera, and the miniDV recorders would record the video and hopefully capture
the shadow of Metis whizzing past at 4km/s. This would appear as the star
winking out for a short period of time that would be dependent on Metis’
physical size and shape.

Initial Occultation Results

The actual
occultation occurred with very little time between the end of twilight and the
passing of Metis’ shadow. Due to the low elevation (+16.8 degrees altitude), the
severe haze up to about 20 degrees of elevation from the Exxon/Mobile oil field
gas plumes burning in the distance, and one encounter with Security, only eleven
stations were successfully pre-pointed with their recorders running. Each
camera’s recording was time stamped in UT with a GPS KIWI OSD system (accuracy
+/-1ms). All eleven of the recordings showed a miss on the asteroid Metis, but
there did appear about a minute before the predicted occultation time, a meteor
about half as bright as the 6th magnitude target star, which had
streaked through the field of view of Station #08. Out of curiosity the other
station’s recordings were reviewed and the same meteor streak was discovered for
stations #09, #10, #11, and #12. Thus five of the eleven stations had recorded
the same meteor, and all of them displayed an easily visible parallax of the
meteor’s position relative to the star HIP 14764. Given the greatest camera
separation of 12.5 km and timing accuracies to 1ms +/-8ms (the duration of one
NTSC video field) an initial attempt was made to reduce these video observations
and gleam as much information about the meteor as possible. What initially came
to mind was the possibility of determining the altitude of the meteor and the
light curve from the ablation. With the GPS position known for each observation
site, and the ability to determine the exact RA and Dec of the meteor at a given
UT time, it was feasible that one could determine a rough orbit, or at a
minimum, the radiant association with any active showers. In addition, the
telescopic nature of the recording is rare in the field of meteor research and
could prove interesting with the 18 arc second resolution available in the
“Mini” camera systems.

Meteor Reduction Results

It took
several weeks by the principle author (SD) of manipulating the raw data before a
satisfactory reduction method was found, having had no past experience in the
meteoric area of data analysis. Once an approach was finalized however, the
actual processing per station was completed in less than an hour.

The first
result concerns the light curve of the meteor. The correlation between intensity
versus time for all five different views/recordings was simply amazing! This
should not be so surprising on reflection, because if all the cameras are
synchronized correctly and properly calibrated, then the results should be
consistent since there is very little change in look aspect or recording
position between the stations. It just wasn’t expected that all five intensity
profiles would lay right over each other as seen in Figure 1. Converting the
meteor track to RA and Dec was also more difficult than it should have been, but
was accomplished for four of the five stations. Station #09 refused to scale
properly so a bit of an average scaling was used to get the track to fall
correctly on the plot in Figure 2. This did not significantly impact the end
result, as Station #08 and Station #12 are the two most spatially separated
stations that one would use to triangulate for altitude. The following is a list
of the initial measured parameters associated with the observation sites and
meteor:

·Approximate peak meteor red magnitude: +6.87

·Time of peak
intensity: 20080912 06:20:44.455 UT +/-8ms

· Station
#08

RA and Dec of peak
intensity: 3h11m13.59s +11 19’ 28.25”

Longitude: -119 37.4429

Latitude: +35 18.0885

Elevation: +331.3m MSL

·Station #12

RA and Dec of peak
intensity: 3h09m54.16s +11 20’ 54.84”

Longitude: -119 40.2197

Latitude: +35 24.3913

Elevation: +158.7m MSL

·Duration: 50 NTSC fields so nearly 1 second long

·Angular
line-of-sight separation between Station #08 peak and Station #12 peak: 19.541’

·
Straight line distance between Station #08 and Station #12: 12.5km

Figure 1: Calibrated light curves of the Metis meteor from the five stations
with a video track.

Figure 2: Meteor positions as observed from the five
stations in stellar coordinates for each video frame.

A back of the envelope calculation placed the line-of-sight range to the
observed track at 2200km or over 800km above the Earth’s surface (locally under
the object), making this initially appear more likely a satellite than a meteor!
At this point, contact was made with US amateurs and professionals in the meteor
community and the co-author (PG) offered to reduce the data further to get a
more accurate estimate of the altitude and potential radiant association.
Calibrating each camera’s field of view from available star positions and using
the intersecting planes solution given the meteor track points from stations
numbered 8 and 12, yielded the following result:

·Apparent angular velocity = 2.33 deg/sec

·Range of the
visible track went from 321 to 345 km - moving away from the observing sites

· Height
of the visible track within the narrow FOV went from 92 km down to 90.5 km
(Figure 3)

· Entry
velocity = 32 km/sec

· Radiant
association = Sporadic

·
The focal plane trailing loss was 2.2 magnitudes

· The
distance fading loss relative to 100 km was 2.6 magnitudes

These
results were consistent when pairing other stations together. Clearly this met
the criteria of a meteor. The back propagation and entry velocity classify this
particular meteoroid as a sporadic.

Figure 3: Meteor height as a function of the line-of-sight
range.

Observing Equipment

As mentioned
earlier, the Metis asteroidal occultation was the “first light” deployment for
the Mighty Mini observing platform. This system consists of one optical
objective from a Tasco Essentials 10x50 binocular mounted to two pieces of PVC
plumbing hardware. The latter provides the optimal spacing given the objective’s
focal length to allow it to focus on a standard high sensitivity security camera
(Supercircuits PC164CEX-2). The camera is placed at the back end of the optical
train that includes an Owl focal reducer screwed onto a 12.7mm spacing video
camera adapter to increase the effective field of view. The system provides a
2.4x3.2 degree FOV with a + 10.2 limiting stellar magnitude under dark sky
conditions at 30 fps video frame rates. The total length of the Mighty Mini with
the camera installed is only 20 cm (8 inches as shown in Figure 4) and thus very
portable and easy to setup at multiple remote sites. A cautionary note is that
the system is optimized for occultation work and despite the extreme magnitude
sensitivity for stationary objects like stars, there can be up to a 4 magnitude
loss for meteors. This is due to the extensive smearing of the meteor across the
high resolution pixels during a single video field integration period (1/60
second).

Figure 4: Mighty Mini video camera and objective lens
system.

A complete
observing platform consists of a Mighty Mini mounted on a MX350 tripod, a Canon
ZR (Models ZR10-ZR300) miniDV camcorder acting as a VCR only, and a 9 cell
battery pack of Duracell AA NiMH 2650mAh batteries as seen in Figure 5. Also
shown is a KIWI OSD which grabs the high accuracy clock signal from the GPS
constellation of satellites for the time stamping/insertion into the video
recording. The complete system can also be operated at prime focus to yield a
smaller FOV that provides higher magnification for lunar occultations of stars.
The complete telescopic portion of the system costs under $100 to build
(excluding the cost of the PC164CEX-2, Owl focal reducer and KIWI OSD) and
weighs less than 10lbs.

Meteor Analysis Conclusions

For an occultation observer, it was amazing that
well over 200 meteor magnitude data points fit so nicely together in both
time and intensity. This provided a reverse verification that the methods of
time stamp placement and determination, as well as the software reduction
tools in use for occultation work, are very efficient and accurate and
transferable to meteor reduction and analysis.

The wider field of view of the Mighty Mini relative
to other occultation camera systems (reducing the apparent angular velocity
of meteors over each pixel) coupled with the sensitivity of the PC164CEX-2
and the low price per system, would seem to make a very handy meteor
observing tool that could be arranged to do triangulation analysis. Due to
the high angular resolution of the narrow field of view cameras, a closer
spacing between observing sites should be tolerable. The loss in spatial
coverage can be partially compensated for by observing at low altitudes
above the horizon as in the Metis collection geometry. This will cover a
larger volume of the air cap at meteor ablation altitudes.

In future asteroidal occultation deployments of
multiple imagers, the video records will be scanned for other meteoric
events. In addition, a test is planned during a future meteor shower to lay
out several stations at varying distances to help determine the optimal
separation distance between camera sites given the high angular resolution
of the Mighty Minis. A large separation baseline of 40 km will provide the
high accuracy results, while the closer stations can be tested to find the
minimal distance acceptable for triangulation processing. If successful,
this could lead to a short baseline and narrow FOV meteor orbit estimation
concept of operations.

September 12, 2008 (9) Metis
Occultation Results

For those who are interested to know the results of the
asteroid shape measurement of that evening, the results were EXCELLENT! Over 30
nearly evenly spaced stations were set up by 20 individuals, and while the 11
deployed by SD as described in this paper were clean misses, David Dunham set up
3 stations using the Mighty Mini, and all 3 got positive measurements! This
makes his three observations the first positive event for the Mighty Mini since
its inception. Figure 6 shows the measurement chords obtained revealing the size
and shape of Metis.

Figure 6: This is a graphical representation of the combined observations of
the Metis occultation.

The lines represent when the target star was visible and breaks in the line
represent when Metis occulted the light from the target star.

By combining the different observations we can measure the shadow an asteroid
casts on the ground,

IOTA discussion
group (you will want to join this to get involved in continuing ongoing
discussions in the field of occultations, the latest equipment, and software
updates, etc. It is an extremely valuable knowledge base)